1. Field of the Invention
This invention relates to sinters of noble metal and a method for the production of sinters of noble metals to be used for the manufacture of shaped products of noble metals abounding in craftsmanship, such as, for example, ornaments of noble metals, artistic craft products, and decorations, and more particularly, to a method for the production of sinters of noble metals which experience only slight shrinkage in the course of sintering and which possess high strength.
2. Description of the Prior Art
The conventional procedure adopted, for the production of sinters of noble metals has been to elevate the temperature of the raw material for sintering slowly in an electric furnace or oven and heat and fire the material over a long time lest the sinter should sustain deformation or fracture such as cracks.
Commercially available clayish compositions for shaping noble metals are obtained by suitably mixing a nobel metal powder, an organic binder, and a solvent as basic materials. When necessary, a surfactant is combined as a mixture promoting agent, along with an oily fat and a plasticizer as agents for preventing the mixture in process of production from adhering to the hands of the artist until the mixture assumes the clayish compositon. The noble metal powder in the clayish composition mainly comprises granulated, shaped, or flat particles having an average particle diameter of 20 .mu.m. As the organic binder, water-soluble cellulose type resin, an acryl type resin, a polyvinyl alcohol type resin, or wax is used at a content in the approximate range of 15-30 wt. %. As the plasticizer, a phthalic ester, a higher fatty acid, a higher fatty ester, or liquid paraffin is used.
Then, the desired sinter of the noble metal is obtained by forming the clayish composition in a prescribed shape, drying the shaped composition, and slowly elevating the temperature of the dried composition in an electric furnace or oven from normal room temperature until it is heated and fired.
The conventional method of production described above is disadvantageous particularly when a plasticizer, a surfactant, an oily fat, and other similar components are mixed in and the mixture is quickly fired. Specifically, the sinter is liable to deform or sustain fracture, such as a crack, owing to quick decomposition, evaporation, combustion, etc. of such organic components. It therefore requires complicated temperature control during firing and inevitably requires the sintering to be continued for a long time (2-10 hours). The cost of the energy consumed in consequence of the protracted firing is enormous. In recent years clayish compositions of noble metal have come to be used in large volume in the field of ornaments and have come to be used particularly in culture classes. The protracted firing time seriously dampens the artist's enthusiasm about creating an ornament. Further, after the electric furnace or oven has been heated to a high temperature for firing a dry shaped composition, its interenal temperature must be returned to normal room temperature by cooling in preparation for firing the next ornament in the subsequent cycle of production, with immense waste of time and energy.
Since the total content of the organic components such as plasticizer, surfactant, and oily fat is high, falling in the range of 15-30 wt. %, the shaped composition in the process of manufacture is markedly shrunken by the sintering and the sinter finally obtained differs from the original artist conceptions. The shaping of the clayish composition therefore must be carried out with allowance for the shrinkage. Since the sinter assumes a porous texture of low strength, its ornamental property tends to be degraded by deformation of the shaped composition under its own weight in the process of firing and deformation that the shaped composition sustains after the firing from a shock or under a load. When the clayish composition is diluted with water and deposited in the form of a thin film on the surface of an object, for example, the thin film of composition sustains numerous cracks due to shrinkage and completely fails to produce the expected ornamental property.
Thus, a strong need is felt for a method for the production of sinters of noble metal which lowers the energy cost by shortening the process of firing after the step of drying and enables formation of the fired composition with minimal shrinkage, thereby maintaining the ornamental property and ensureing high strength.